Screen changers of many types have been proposed in the prior art; for example, the rotating screens of U.S. Pat. No. 3,455,357 to Zink of July 15, 1909, wherein there are ports and flow passages in the body of the changer for venting and pre-filling each successive clean screen. Similarly in the rectilinear sliding screens of U.S. Pat. No. 3,804,758 to Cooper of Apr. 16, 1974 there are conduits in the body member for venting and pre-filling each successive clean screen, the pre-filling being initiated by a manually operated valve before the clean screen commences to move.